In a solenoid actuator which operates hydraulic equipment through the linear motion of a shaft introduces, for example, working oil in the hydraulic equipment into the actuator to lubricate bearings supporting the shaft or realize a preferable balance of pressures acting on a front face and a rear face of the bearings.
When working oil containing a contaminant such as abrasion powder generated in the hydraulic equipment is introduced into the solenoid actuator, the contaminant tends to be deposited in a strong magnetic field portion of the actuator. The contaminant deposited in the strong magnetic field portion shaft adversely affects the operation of the solenoid actuator by weakening a thrust which the solenoid exerts on the shaft or increasing a sliding resistance of the shaft with respect to the bearings.
JP2001-317653A, published by the Japan Paten Office in 2001, proposes a seal structure to prevent the contaminant from invading a solenoid actuator. The solenoid actuator is configured to displace a plunger fixed to a shaft by a magnetic force of a solenoid together with the shaft. The shaft is supported by bearings disposed on a front side and a rear side of the plunger so as to be free to slide axially. By providing ring-shaped seal members on an inner side of the bearings with respect to an axial direction of the shaft, the contaminant is prevented from invading a plunger chamber formed between the bearings.
In the interior of the actuator, a rear chamber is formed on a rear side of one of the bearings which is located farther from the hydraulic equipment. A shaft-penetrating passage which introduces working oil from the hydraulic equipment to the rear chamber is formed through the shaft.